Natural gas is commonly used as a fuel. The gas is generally under considerable pressure when it is extracted from the gas reservoir and this pressure is maintained in a high pressure core of a gas transmission and distribution system. The gas pressure within the transmission system (at least in the UK) is typically in the region of 72 bar. Gas at this pressure is unsuitable for distribution to consumers due to the potential safety hazards of working at this pressure and consequently the transmission and distribution system reduces the pressure typically in a first stage from 72 bar to an intermediate pressure of around 32 bar and subsequently to less than 1 bar compared to atmospheric pressure at the delivery point. Thus the transmission and distribution systems can be regarded as having a high pressure portion operating at around 72 bar, a medium pressure portion operating at around 32 bar, and a low pressure portion operating between 7 bar and eventually at around 1 bar. At the transition from one portion to another portion the gas pressure needs to be reduced and this is performed by a series of pressure reducing valves at pressure reduction stations. Other (but similar) pressure schemes within a distribution network are known.
The expanding gas changes volume, and as it does so it performs mechanical work. The gas expansion process is essentially adiabatic and consequently the energy for the mechanical work done by the expanding gas is removed from the thermal energy carried by the gas and consequently the gas cools as it expands. The cooling of the gas may result in the formation of ice and/or complex hydrates in and around the gas carrying pipes which can damage control valves within the gas distribution system and cause freezing of the ground around the pipe which in turn may cause a permafrost zone to be created which could give rise to distortion or ground heave around the pipe.